1. Field of the Invention
The invention relates to a method and a device for preparing process gases for heat treatments of metallic materials/workpieces in industrial furnaces, wherein the heatable process gases include a treatment medium as a protective gas and/or for example a reaction gas.
2. Description of the Related Art
In general, a process gas for the heat treatment of metallic materials/workpieces in industrial furnaces is understood by a person skilled in the art to be a treatment medium such as for example a protective gas containing carbon monoxide, hydrogen and nitrogen, carbon dioxide oxygen and/or steam, and/or for example a reaction gas containing hydrocarbons for “carburisation processes”, which relate to the carburising or carbonitriding of metallic materials/workpieces.
Thus, in one of the steps of carburisation for example, a gas containing hydrocarbons is added to a process gas reacting in the treatment chamber to create the requisite furnace atmosphere. In this process, the individual components of the process gas are intended to create a controllable state of equilibrium in the furnace atmosphere so that the carbon is able to be transferred from the gas atmosphere to the metallic materials/workpieces in a manner that is both controlled and reproducible. Automatic control of processes of this kind is assured by regulation of the C level, such as is described in DE 29 09 978 and has become successfully established in industrial practice for the heat treatment of metallic materials/workpieces. However, the control of the C level solved so advantageously here fails to make use of catalytically usable potential with regard to modern requirements.
Thus for example, in his report entitled “Gas mixtures fed hot into the furnace chamber as the atmosphere for heat treatment of steel”, (HTM 30 (1975) Vol. 2, p. 107-) W. Goring had already suggested using a protective gas retort with catalyst bed integrated in the industrial furnace to feed hot process gas into the treatment chamber, regardless of the temperature there, as a way to speed up the level of activation of a furnace atmosphere. For the purposes of modern specifications, this method is associated with a number of drawbacks, because it requires constant replenishment with fresh gas, that is to say regulation is effected by enriching the gas, and harmful emissions must be contended with.
The use of catalysts is also described in other documents, such as for example DE 36 32577, DE 38 88 814, DE 40 05 710, DE 691 33356, and DE 44 16 469.
The following text discloses the treatment of metals in a carburised atmosphere                in accordance with GB 1,069,531,        in accordance with U.S. Pat. No. 3,620,518 for the treatment of workpieces in hardening furnaces having a catalyst lining of nickel oxide, which has been applied to the ceramic interior wall and increases the available surface area,        in accordance with U.S. Pat. No. 4,294,436 with a furnace for heat treatment of metal parts with a protective atmosphere in furnaces having catalytic walls of Ni,        in accordance with U.S. Pat. No. 5,645,808 for catalytic oxidation with carbon compounds in gas streams, and        in accordance with US 2006/0081567 with plasma-supported workpiece treatment, and        in accordance with JP 62199761.        
In order to improve the process effect of the gas feed in the abovementioned industrial furnaces, it has already been suggested in DE 10 2008 029 001.7-45 to tailor the supply of hydrocarbon to specific carburisation requirements so as to economise on protective gas and reduce heat energy losses, to adjust the C potential in the protective gas and prevent uncontrollable and/or undesirable reactions. This resulted in the creation of a new protective gas recirculation system for gas carburisation. In this, the components carbon dioxide, oxygen and steam react with a supplied hydrocarbon in a processing area of the treatment chamber of a species-related industrial furnace to yield carbon monoxide and hydrogen again, in this case catalytically. In this way, previously “used” protective gas, that is to say a protective gas with a low C potential, may be advantageously reprocessed. The C potential is adjusted in the processing area of the treatment chamber. The “processed” protective gas may then be fed back into the treatment chamber at one or more points, thus establishing a truly circular process for gas carburisation.
According to this new method                the components carbon dioxide, oxygen and steam react with a fed supply of a hydrocarbon as the reagent gas to yield carbon monoxide and hydrogen in a processing area equipped with catalyst bed in an industrial furnace,        the protective gas has elevated C potential, and the C potential is adjusted,        the catalytic reaction is accelerated, and        the protective gas processed in this way is returned to the treatment chamber in a recirculation system.        
The purpose of this was to improve the process of uniform carburisation and enable costs for process gas to be reduced further.
However, more extensive research was needed in order to ensure even more reliable and reproducible heat treatment for industrial furnace operators, because the method described above requires the treatment chamber and the heating chamber to be as impermeable as possible, and reaction temperatures in the heating chamber do not exceed for example 850-950° C.
In this context, the quality requirements for case hardening had to be analysed again, particularly with respect to parameters such as                case hardening depth/carburisation depth,        surface hardness/surface carbon content,        perlite/troostite seam,        residual austenite content,        carbide formation,        surface oxidation depth as well as        dimensional and shape changes and        core hardnessif they were to be correlated even indirectly with the degree of carburisation. In this case, the depth of carburisation and the carbon concentration were both significant factors.        
In the existing industrial carburisation methods, such as gas carburisation in atmosphere furnaces and low-pressure carburising in vacuum furnaces, the objective is one that is familiar to those skilled in the art: all parts of the workpieces in a batch must be carburised with total uniformity, to the same C content and the same carburisation depth.
With gas carburisation, in which the furnace atmosphere is adjustable via equilibrium reactions, this may be achieved more effectively than by non-equilibrium carburisation using hydrocarbons.
Accordingly, gas carburisation, that is atmospheric gas carburisation, is the preferred process.
In this process, the following known, various process steps are performed, it is essential to take all of them into account to ensure reproducible, even carburisation:                1. The gas reactions for creating the carburising gas components in the atmosphere.        2. Gas phase homogenisation for transporting the carbon-containing molecules in the gas phase and to the part to be carburised.        3. Diffusion transport, that is to say transport of the carbon-containing molecules by the flow boundary layer to the surface of the part.        4. Dissociation and adsorption relating to splitting of the molecules on the surface of the part.        5. Absorption, that is to say the uptake of the carbon by the part surface.        6. Diffusion as the means of transporting the carbon into the part.        
As has already been described in the outcome in DE 10 2008 029 001.7-45, the decisive reactions for carburisation in the carburisation atmosphere are:CH4C+2H2  Methane dissociation2COC+CO2  Boudouard reactionCO+H2C+H2O  Heterogeneous water-gas reactionCH4+CO22CO+2H2  Enrichment reaction 1CH4+H2OCO+3H2  Enrichment reaction 2
In order to build on the advance represented by DE 10 2008 029 001.7-45 with regard to the prior art, it is important to influence the kinetics and also the direction of these reactions, because they depend to a large degree on the temperature which—as was explained previously—is regularly limited to 850-950° C., but are not enabled at temperatures significantly below this.
Since the transportation of the carbon carrier is usually effected through forced convection, the powerful circulation of the atmosphere within the heating chamber helps to ensure that the carbon carriers are thoroughly mixed and the flow thereof is then directed towards the part.
Thus, for example, the following relationship is known to apply for mass transfer when the atmospheric flow is directed towards a flat workpiece panel
            β      L        =          0.664      ×                                    V            ·            L                    ν                    ×                        ν          D                3            ×              D        L              ,wherein                a coefficient of diffusion is represented by D;        a length of the part to which flow is directed is represented by L;        a flow speed is represented by V; and        a kinematic viscosity is represented by ν.        
Accordingly, as the flow speed increases the effective coefficient of mass transfer β also becomes larger, and it is this relationship that must be used even more efficiently.
The known relationship to the effect that the speed of diffusion through the flow boundary layer is essential and cannot be influenced by changing the flow speed had to be studied further.
In this regard, it is the magnitude of the coefficient of diffusion in the gas that is decisive, and this is critically dependent on temperature and pressure. In an initial approximation—also known—, doubling the pressure halves the diffusion coefficient.
The thickness of the boundary layer may be determined using the relationship familiar to those skilled in the art
      δ    ⁡          (      x      )        =      4.64    ⁢                            ν          ·          x                          V          ∞                    where                thickness of the boundary layer is represented by δ(x);        distance from the edge of the part is represented by x; and        speed of the gas farther from the part is represented by V∞.        
It may be observed that increasing the speed of the gas reduces the thickness of the boundary layer, which in turn shortens the transport time to the surface. Use of this relationship must be improved as well.
Finally, the splitting reaction of the carbon monoxide on the part according to the known equation
      C    ⁢                  ⁢    O    ⇒      C    +                  1        2            ⁢              O        2            is also the underlying reaction for transferring carbon for gas carburisation in CO/H mixtures, which, besides still other reactions, enables the carburisation effect of the atmosphere to take place continuously, as is shown in FIG. 1, which illustrates known findings.
In order to arrive at advanced solutions proceeding from this known basis, the creative approach had to be applied and exploited in a technologically new way, in particular that                the speed of carbon transfer depends on the property of the atmosphere, and        the oxygen generated during splitting must be bound and then removed by convection.        
Since hydrogen is needed for this, the speed of dissociation of the carbon monoxide in the presence of a sufficiently large quantity of hydrogen becomes the determining parameter.
The speed at which the carbon is absorbed by the workpiece surface in turn depends on the difference between the carbon activities in the atmosphere and in the part. This means, if the carbon activity in the gas is greater than in the part, the net effect is a transfer of the carbon to the workpiece surface.
In practice, this difference may be characterized in pure iron and unalloyed steel by the difference between the C potential and the carbon content in the workpiece surface, wherein the diffusion of the carbon can be described by Fick's laws, which will not be further elaborated on here.
Accordingly, a new inventive task must address the fact that diffusion depends on the temperature and the progression of the concentration of carbon C in material having depth x.